Protein aerogels as functional ingredients able to replace fat and modulate lipid digestion

Introduction The direct relation between saturated fat consumption and chronic diseases such as cardiovascular diseases, diabetes type II and obesity is nowadays well-established. However, fat substitution in foods is not simple, due to the important structural and sensorial properties of solid fat, which are hardly replicated by liquid oil. Nevertheless, oil can be structured into semi-solid materials (oleogels) by different “oleogelation” strategies. Oleogels not only mimic the structural properties of fats, but have been also shown to be useful in the modulation of lipid digestion. The aerogel-template approach, is a recent oleogelation strategy, based on the ability of aerogels to absorb oil in their porous network. In particular, food-grade protein aerogel particles have shown the peculiar ability to structure huge oil amounts into plastic systems presenting the mechanical properties of traditional fats. The aim of the present study was to assess the effect of aerogel-template oleogelation on lipid digestibility and to investigate the possibility to use aerogel-templated oleogels in the preparation of low-saturated fat cocoa creams. Experimental Methods Whey protein (WP) aerogel particles were prepared by grinding a heat-set WP hydrogel (20% w/w, pH=5.7), which was then subjected to ethanol exchange and supercritical-CO2 drying (SCD). Oleogels were than obtained by absorption of sunflower oil (SO) into aerogel particles. Lipid digestibility of the oleogel containing 80% (w/w) SO and 20% WP aerogel (w/w) was assessed by in vitro digestion, according to the INFOGEST protocol. Lipid digestibility was expressed as free fatty acids (FFA %), assessed by pH-stat method, i.e., by measuring the volume of NaOH (0.25 M) required to maintain the pH at 8.00 during digestion occurring in the small intestine. The choice of using pH 8.00 instead of 7.00 was based on the technical specifications of the used lipase. The digestate samples were analyzed by using dynamic light scattering (DLS) and confocal microscopy. The WP aerogel particles were then used to prepare cocoa creams containing sunflower oil (SO), icing sugar and cocoa powder. Different oil amounts were tested (40-65% w/w), while maintaining constant the ratio among the dried ingredients (WP aerogel:sugar:cocoa =1.5:5:1). Additional control samples were prepared by using native WP. The obtained creams were analyzed for oil release and rheological properties and compared to cocoa spreads available on the market. Results and discussion WP aerogel particles were used to structure SO into an oleogel, whose digestibility was then assessed. The lipid digestibility of SO and of the oleogel resulted respectively of 70% and 80%. These results can be attributed to the ability of aerogel protein particles to improve the emulsification of oil in the intestinal digestive mixture, leading to an enhanced activity of lipolytic enzymes. DLS, in fact, evidenced that the lipidic micelles formed during intestinal digestion of the oleogel resulted significantly smaller than those formed during SO digestion. This is probably attributable to the surface activity of WP aerogels, which are able to cover and stabilize the oil droplets in the digestive mixture. The applicability of WP aerogel particles as key ingredients for the preparation of low-saturated fat cocoa creams was then demonstrated, combining WP aerogel particles with SO in presence of sugar and cocoa powder. Native WP did not show oil structuring ability, leading to liquid-like cocoa creams, showing an apparent viscosity lower than 2 Pa∙s (50 1/s) and evident oil release upon resting at room temperature. By contrast, aerogel particles produced thicker creams, showing no flow under gravity, a significantly higher viscosity (50 Pa∙s), and no oil release under standard storage conditions. This was attributed to the modifications undergone by WP during conversion into porous aerogel particles. The range of rheological properties covered by the WP aerogel cocoa creams resulted comparable with a wide variety of commercial products (e.g. sauces and batters). Conclusions This work demonstrates the potentialities of WP aerogel particles as oil structuring agents, exploitable in the formulation of healthier food products with a reduced amount of saturated fatty acids. Such formulation strategy would not compromise the lipolytic action during digestion, making aerogels suitable carriers of bioactive molecules in the gastrointestinal tract.